What are the differences between the 'base' of various Linux distributions?
A distribution distributes packages. Each base has its own set of packages, compilation flags or "package configuration", optimizations, release schedule, and overall goals.
Besides technical reasons, people may not like some "bases" for political reasons too.
How easy is it to switch back to windows?
If you have a flash drive and an external disk you can boot into Rescuezilla (a disk cloning/backup/restore distribution) using the flash drive, save/backup your OS disk as an image in the external disk, and restore it as needed. After restoration, you will have your OS disk as it was at the time you saved it.
